[0001] This invention relates to ignition systems of the kind including a first capacitor,
a circuit for charging the first capacitor and a circuit for applying the voltage
on the first capacitor to primary windings of a transformer.
[0002] Conventional ignition systems employ a capacitor charged from a voltage source. When
the charge on the capacitor has reached the necessary level, a switch is closed and
the charge is applied to the primary windings of a transformer. The transformer acts
to step up the voltage, the secondary windings being connected to a cable extending
to an igniter mounted in a burner or the like. The igniter is often located some distance
from the ignition system and is connected to it by a high voltage coaxial cable. Long
cables of this kind act as transmission lines and seriously attenuate the fast pulses
sent to the igniter electrodes. Furthermore, the charging of the self-capacitance
of the cable absorbs much of the available energy and may result in insufficient energy
at the igniter electrodes to produce reliable ignition.
[0003] It is an object of the present invention to provide an improved ignition system.
[0004] According to the present invention there is provided an ignition system of the above-specified
kind, characterised in that the system includes a second capacitor connected to be
charged by the voltage on the secondary windings of the transformer, and that the
system is arranged such that the voltage on the second capacitor increases progressively
each time the charge on the first capacitor is applied to the transformer until the
charge on the second capacitor is sufficient to cause discharge at the igniter.
[0005] The circuit for applying the voltage on the first capacitor to the primary windings
preferably includes a switch. The system preferably includes a cable having capacitance
extending between the second capacitor and the igniter, the charge on the secondary
winding being supplied to charge both the second capacitor and the capacitance of
the cable. The ignition system preferably includes a rectifier circuit between the
secondary windings and the second capacitor.
[0006] A conventional ignition system and one according to the present invention, will now
be described, by way of example, with reference to the accompanying drawings, in which:
- Figure 1
- is a circuit diagram of the conventional system; and
- Figure 2
- is a circuit diagram of the system of the present invention.
[0007] With reference first to Figure 1, the conventional system has a storage capacitor
1 connected across a voltage source 2 via a resistor 3 and a diode 4. Opposite plates
of the capacitor 1 are connected across opposite ends of a primary winding 5 of a
transformer 6 via a series-connected switch 7, which may be a mechanical or solid
state switch, such as a thyristor. The secondary winding 8 of the transformer 6 is
connected across the electrodes 10 of an igniter 11 via a coaxial cable 12. Figure
1 shows the electrical equivalent circuit of the cable 12, which comprises three series-connected
inductors 13 in both conductors 14, and three capacitors 15 connected in parallel
between the two conductors at junctions between the inductors.
[0008] With reference now to Figure 2, there is shown an ignition system according to the
present invention. Components in the circuit of Figure 2 equivalent to those in Figure
1 are given the same reference numeral with the addition of a prime '. That part of
the system on the input/primary side of the transformer 6' is identical to that of
Figure 1, except that the capacitor 1' is generally smaller than that in the conventional
system, so this part will not be described again here. The part of the system on the
output/secondary side of the transformer 6' differs from the equivalent part in Figure
1 in that a secondary capacitor 20 is connected across the secondary winding 8' via
a rectifier circuit 21 and at the input end of the cable 12'. The rectifier circuit
21 is a half-wave device but could include a full-wave circuit so that it acts bidirectionally
to recover more efficiently energy that may be lost in the "fly back" or ringing of
the transformer 6'. The secondary capacitor 20 is connected across the two conductors
14' at the input of the cable 12'.
[0009] This circuit does not discharge sparks at the igniter 11' each time that the switch
7' is closed, but only after several cycles of charging and discharging the primary
capacitor 1'. When the switch 7' is closed, energy is transferred to the secondary
circuit of the transformer 6', as before, but the energy is applied across the secondary
capacitor 20. The rectifier circuit 21 prevents the capacitor 20 discharging through
the secondary windings 8' when the voltage across the windings drops, so the charge
in the capacitor is built up each time the switch 7' is closed. As the voltage builds
up on the capacitor 20 it also builds up on the distributed capacitance 15' in the
cable 12', which effectively forms a part of the secondary capacitor. Fast voltage
pulses no longer travel down the cable 12', so the available voltage is not attenuated.
Each time that the switch 7' is closed, the voltage on the capacitance 20 and 15'
of the secondary circuit will increase progressively. When this voltage exceeds the
breakdown voltage of the igniter 11', the charge on the capacitances 20 and 15 is
discharged across the igniter electrodes 10' to ignite the surrounding fuel/air mixture.
[0010] The system of the present invention gives a very reliable discharge of sparks at
the end of a highly capacitive cable. It can be seen that the system progressively
increases voltage until discharge occurs, in contrast with previous systems where
the voltage applied is of a set value and may be insufficient to cause ignition in
some circumstances. The present invention is, therefore, particularly useful for igniting
fuel mixtures with a high dielectric strength, which are reluctant to ionize.
[0011] The circuit can be varied in various ways. For example, the primary circuit may be
of various different kinds. Also, the secondary capacitor need not be located at the
input end of the cable but could be located at some point along the cable, or at the
igniter electrodes themselves.
1. An ignition system including a first capacitor (1'), a circuit (2', 3', 4') for charging
the first capacitor, and a circuit (7') for applying the voltage on the first capacitor
(1') to primary windings (5') of a transformer (6'), characterised in that the system
includes a second capacitor (20) connected to be charged by the voltage on the secondary
windings (8') of the transformer (6'), and that the system is arranged such that the
voltage on the second capacitor (20) increases progressively each time the charge
on the first capacitor (1') is applied to the transformer (6') until the charge on
the second capacitor (20) is sufficient to cause discharge at the igniter (11').
2. An ignition system according to Claim 1, characterised in that the circuit for applying
the voltage on the first capacitor (1') to the primary windings (5') includes a switch
(7').
3. An ignition system according to Claim 1 or 2, characterised in that the system includes
a cable (12') having capacitance extending between the second capacitor (20) and the
igniter (11'), and that the charge on the secondary winding (8') is supplied to charge
both the second capacitor (20) and the capacitance (15') of the cable (12').
4. An ignition system according to any one of the preceding claims, characterised in
that the system includes a rectifier circuit (21) between the secondary windings (8')
and the second capacitor (20).
